Cobalt(III) Complexes of a Stereospecific Ligand Derived from (S)-Proline

Abstract

The quadridentate ligand, (2S,2'S)-1,1'-(ethane-1,2-diyl)bis(pyrrolidine-2-carboxylic acid) dihydrochloride (H₂pren,2HCl), has been derived from (S)-proline.

The preparations of the cobalt(III) complexes Na [Co(pren)(CO₃)],3H₂O, [CO(pren)(H₂O)₂] ClO₄,- 2H₂O and [Co(pren)(H₂O)Cl],1·5H₂O are described. All exist as cis-α geometrical isomers. ¹³C N.M.R. and absorption spectral measurements confirm the cis-α arrangement. The complex cis-α- [Co(pren)(H₂O)Cl],1·5H₂O rapidly aquates with retention of configuration.

The compounds having actual or near C₂ symmetry exhibit a strong negative c.d. band under the ¹A_1g → ¹T_1g cubic absorption band which is due to the ¹A₁ →¹E tetragonal component, consistent with the Δ absolute configuration imposed by the (S) configuration of the asymmetric carbon atom in the (S)-proline moiety.

The o.r.d. spectrum of cis-α-Na [Co(pren)CO₃],3H₂O in aqueous solution has a negative Cotton effect at low energy as expected for the Δ absolute configuration.